The present invention relates to a system for automatically effecting speed change in power transmission from an internal combustion engine or the like. By utilizing the gear ratio change action of a planetary gear mechanism.
More specifically, the invention relates to an automatic transmission in which a planetary gear mechanism is mounted on a drive shaft to permit reduction of the length dimension of the drive shaft, mainly for the purpose of making the entire system more compact.
In general, a motorcycle has a body which is considerably small compared to a common passenger car or other such vehicle, and the space required for assembling necessary parts and devices is limited. Particularly, in small motorcycles such as "mopeds" which are designed to have a reduced weight and size and a simplified driving operation, the smaller the size of the motorcycle, the smaller the space available for assembling necessary parts and devices. In order to cope with the limitation upon available space, it is necessary to make the apparatus to be accommodated in such a space more compact. Further, this requirement may be applied not only to motorcycles, but also to common vehicles and other machines and apparatuses wherein a compact arrangement of a number of parts and devices is necessary to avoid excessive space consumption.
In the aforementioned small motorcycles or similar vehicles, it is necessary to provide a construction which permits ready driving operation even by one who is not skilled in driving techniques, in addition to minimizing the number of parts so as to simplify construction. Because a construction in which a gear ratio change is effected by a pedalling operation requires much skill for expert handling, it is desirable to employ an automatic transmission in which the gear ratio of an output shaft for rotating the drive wheel is automatically changed with the increase of the engine rpm. The automatic transmission usually has an assembly including a number of parts such as clutch means, and with an increase of the number of component parts, the space for assembling the parts is correspondingly increased, leading to a size increase for the entire system. For example, in an automatic transmission which comprises a centrifugal clutch mounted on a drive shaft, a plurality of first gears coupled to inner and outer members of the clutch, and a plurality of second gears provided on a counter shaft which are different in diameter from the first gears, the first gears are naturally spaced apart from each other in the axial direction because they must be coupled to the inner and outer members of the clutch, and the second gears in mesh with the first gears are also axially spaced apart from each other. Therefore, the length dimension of the shafts is increased. In addition, the counter shaft for taking out the transmitted power has to be provided in parallel with the drive shaft, which inevitably causes a further increase in the size of the system. Increased size of the system, however, must be avoided, which results in one of the problems associated with the design of the automatic transmission. A compact automatic transmission is useful not only for motorcycles but also for other machines and apparatuses, i.e., it can also be employed for systems other than motorcycles.
In view of the foregoing problems, the present invention provides a transmission system extending from a drive shaft to an output shaft as an automatic transmission, such automatic transmission being compact so as to be readily mounted even on a very small motorcycle.